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Why are some species so invasive? [Invasive Species, Part 1]

If you drive longer than five miles in the South, you’ll undoubtedly see an abandoned lot covered in kudzu. Up until the 1950s, farmers transplanted the fast-growing vine from Asia to the U.S. to stop soil erosion along roads. Individual kudzu vines can grow more than 100 feet per growing season, easily spreading over the ground. At first, it seemed like the perfect plant to quickly cover exposed soil and stop erosion. But then… it didn’t stop growing. Turns out, kudzu grows so quickly that it displaces native plants, and it climbs and kills trees by shading them out! One study estimates that kudzu causes $500 million per year in lost productivity. After covering 7.4 million acres in the southern U.S. (an area about the size of Maryland), kudzu has earned its nickname as “the Weed that Ate the South”. As a kid growing up in Mississippi, I remember thinking that the kudzu-covered trees along the roads looked like Swamp-Things rising out of the vines. Others find even weirder shapes in these kudzu-covered landscapes.

Despite the fun shapes it makes, kudzu and other invasive species can cause many problems. A quick glance at the Species Survival Commission’s 100 of the World’s Worst Invasive Alien Species shows that invading organisms can be plants, fish, amphibians, reptiles, mammals, insects, and even microbes. Water hyacinth, a floating plant, grows thick enough in rivers to block boat traffic, causing economic problems. Miconia is an invasive tree that has spread to dominate over half of the island of Tahiti. The roots of Miconia are not as deep as the native trees, so they offer less protection from landslides on the island. Ever since the Southern House Mosquito was introduced to the Hawaiian Islands, an invasive strain of malaria has infected vulnerable native birds. But the list of harmful invasive species goes on to include the crab-killing Yellow Crazy Ant, the bird-killing Brown Tree Snake, the egg-eating Western Mosquito Fish, and many other animals and microbes which are all implicated in damaging ecosystems.

Clearly invasive species can cause problems, but how do they grow so fast? What kinds of problems do they cause? How do we deal with them? This blog is the first installment of a series focusing on invasive species. I’ll kick off the series by focusing on plants, and talking about two ideas that help explain how invasive plants can so rapidly conquer new territory.

Escaping the Enemy

Invasive species can sometimes spread very rapidly by escaping their “enemies.” Sometimes, a plant like kudzu escapes its enemies if it colonizes a new region before its predators or pathogens can follow. When ecologists from Cornell University looked at over 4,000 plants from Europe introduced to the U.S, they found that these plants had escaped an average of 84% of the plant’s native fungal pathogens. Even more interesting, the researchers found that some of the most aggressive invaders were also the same plants that “escaped” the most pathogens! So one reason invasive plants can grow so fast is because they’re not being infected all the time!

Developing Novel Weapons

On top of escaping their enemies, some invasive plants resort to straight-up chemical warfare! Many plants produce substances called allelopathic chemicals that are toxic to other plants The chemical warfare waged by invasive species is particularly effective because most of the native plants have never before encountered the toxin! Knapweed, a European plant that became invasive in the U.S., produces an alleopathic toxin on its roots. When ecologists from the University of Montana studied knapweed, they found that it was almost twice as toxic to grasses from the U.S. compared to grasses from Europe that have evolved alongside the toxic knapweed. This result probably means that grasses in the U.S. were only recently exposed to knapweed’s toxins and haven’t yet had time to evolve a defense. My cheesy warfare analogies weren’t for naught. Ecologists actually call this idea the novel weapons hypothesis. Who knew plants could be so violent?

The War on Invasive Species

These studies shed light on the theory behind why some plants become invasive, but much more research is being done to understand how invasive species impact the environment. In the next few posts in our series, we will tell you the interesting stories behind some important invaders and what problems they can cause. Most importantly, we will talk about how we can keep invasive species from gaining more territory.

Jeffery B. Cannon is a doctoral candidate at the University of Georgia researching how disturbances such as wind and fire shape forest ecosystems. He loves science, the outdoors, and enjoys running and brewing beer. You can connect with Jeff on Twitter @JefferyBCannon or by email: jbc165@gmail.com